1. Technical Field
The present invention relates generally to apparatus for determining odor levels in odorized gas streams.
2. Description of the Related Art
Federal and state regulations require that combustible gases (e.g., natural gas) in transmission lines be odorized to provide a quick and easy means for detecting the presence of gases. Odorization involves adding small amounts of a chemical with a distinctive odor to the gas stream. This odor facilitates leak detection and provides an early warning of potentially unsafe conditions.
The odor must be readily detectable by a person with a normal sense of smell when gas is present at a given concentration in air. The concentration is typically specified as a certain percentage of the lower explosive limit (LEL) of the gas.
Federal and state odorizing regulations require gas operators to conduct periodic sampling at distribution sites to assure proper odor levels. Portable instruments, sometimes referred to as odorometers, have been devised to determine gas odor levels using olfactory testing. These instruments are available from a number of manufacturers including Bacharach, Inc. and Heath Consultants, Inc.
In one known device, gas to be analyzed enters the instrument through a gas intake port. The gas passes through a low pressure regulator and then a flow adjustment valve, which is operated by a user. The gas then enters a sensor chamber, which measures gas flow. A signal from the sensor chamber is processed, converted to a digital signal and fed to a liquid crystal display (LCD). The gas from the sensor chamber is then mixed with air drawn in by an air blower. The air-gas mixture then exits the device through an exhaust port, where it can be sniffed by a user.
One test that is commonly performed with such a device is known as a threshold odorant test (TOT). This test is used to determine a threshold gas-air concentration at which odor is barely detectable. In this use, the operator slowly opens the flow adjustment valve, allowing the gas sample to enter the instrument, while at the same time breathing normally with his or her nose placed close to the exhaust port. When odor in the sample is detected, the operator depresses a read button to observe a reading on the LCD corresponding to gas flow. A look-up chart is then used to convert the reading into a percent gas concentration, which is then hand-written somewhere for recordation.
Another use of the device is to determine odor characteristics at given gas-air concentrations. In this use, a gas/air mixture at a preset concentration is sniffed by various persons who then categorize the odor level as "absent," "barely detectable," "readily detectable," "strong" or "obnoxious."
A significant problem with these known devices it that they fail to provide accurate and repeatable gas concentration readings. Except for the LCD output, the instruments are substantially analog devices and lack adequate control means to ensure steady operation. For instance, the fans or blowers used to draw air through the devices cannot be sufficiently controlled to ensure constant speed, which is critical in determining gas concentration.
Furthermore, the known devices fail to provide adequate mixing of the sample gas and air. Consequently, the gas-air mixtures are not uniformly mixed, resulting in testing inaccuracies.
In addition, known devices are not equipped with recording means for storing and/or analyzing test data. Consequently, users must hand-write the data somewhere and use a look-up table to cross-reference data with gas concentration values. This is a potential source for further errors.